@Dakotah. In terms of combat, I have found @tesha, and @carmell very useful. Also, race stats and traits can help as well. Useful traits for priests are:

Devout--faster replenishment of devotionMind warden--Makes your contemplate ability "free"Quick-Witted--faster equlibrium recovery--This one is very important. Everything priests do requires equilibrium--except for the smite--This will help you to attack faster.

As far as useful organizations, I think it is a good idea to learn as much as possible about the Bloodsworn. One of the best decisions I ever made was joining the Votaries of Light--Lady Aurora's pre-order clan. If you would like to know more about it, speak with me in game. Lord Deucalion also has a pre-order clan that is called Kindling.

Also I have heard that the Dawnguard is a useful source for good information on combat.